Naturally brewed soy sauce is produced by putting koji into a charging device having an appropriate amount of salt water and fermenting this koji. However, koji is lighter than salt water, i.e. it has a lower specific gravity, and it also includes fat constituents, such as soybean, and these constituents have water repelling properties and accordingly water absorption is difficult, thus resulting in the floating of a thick layer of koji on the salt water. In such a case, it is necessary to agitate to make good contact between the koji and salt water; however, when a mechanical device is used to perform the agitation, it is inevitable that the agitation effect becomes excessive resulting in a moromi having excess viscosity. Here and hereinafter moromi means the mixture of koji and salt water and koji means the starter prepared from the mixture of steamed soybean and burned wheat, inoculated with spores of Aspergillus oryzae and permitted to develop mycelia.
Therefore, the usual manner of making soy sauce involves a very long period for the water absorption of the koji such as about one year according to a prior natural brewing method.
Shortening of the production period has been a great problem so far in the art. Since agitation of moromi cannot be done effectively, when using a large capacity tank, carbon dioxide gas is not discharged out of the system between the intermediate portion and the bottom portion; however, if the CO.sub.2 is not discharged, putrefaction may arise. Accordingly, in accordance with recent techniques it has been impossible to produce soy sauce in quantities by means of a large capacity tank such as 200 or 300 kl, and moreover since the prior soy sauce brewing system is an open-system, volatile constituents such as alcohol, sugar, fragrance constituent and so forth volatilize out of the system, thus causing deterioration of quality and reduction of yield. Such drawbacks have been inevitable in the prior art.